Attention Scholars: Always place your Fairy!
I'm sure every Scholar has noticed how often your Fairy will just sit idly by as you or a party member dies. The solution? Use the Place command. For whatever reason, if the Fairy is placed, she will immediately start casting as soon as someone drops under 80% and will chain-cast Embrace nearly twice as quickly as if she's on Heel.
Go jump into an AK/CM and give it a try. It makes a world of difference.
Just do what you think is appropriate and most efficient for you
While I do agree that when she is placed somewhere safe, the result is 10 times better (you can move around and she still focus on the tank) I don't agree for every situation
you mentioned AK and I hate placing my fairy during the Demon Wall
I will always keep her on heel for that fight, the 2 other bosses I place her.
For Ifrit and Garuda I also prefer having her on heel (because Ifrit pushes her and Garuda and the tornados change places all the time)
No idea about Titan, haven't done it
So don't place your fairy if it will involve having to change place every 10 seconds or recast her because she died. If there is a relatively safe spot near the tank, go for it.